Our invention belongs to the art of fabricating semiconductor devices such as pn-junction diodes, Schottky-barrier diodes, transistors and thyristors. More particularly, our invention pertains to a method of diffusing gold into semiconductor substrates in the manufacture of such semiconductor devices with a view to the improvement of their switching characteristic.
The diffusion of gold into semiconductor substrates represents a familiar technique in the fabrication of high-speed rectifier diodes and other semiconductor devices. It aims at the improvement of the switching speed of the semiconductor devices through reduction of the lifetime of the minority carriers. According to a typical conventional method of gold diffusion, as adapted for the manufacture of pn-junction diodes, for example, a layer of gold is formed by vacuum deposition or by plating on one of the faces of a semiconductor wafer following the creation of a p- or n-type region in the wafer by impurity diffusion. Then the wafer with the gold layer thereon is heated to cause gold diffusion into the wafer by way of a lifetime killer.
This known method of gold diffusion has had a problem arising from the fact that the temperature at which gold is diffused is in inverse proportion to the resulting lifetime of the minority carriers. The higher the temperature, the shorter the minority carrier lifetime. Consequently, gold has had to be diffused at as high a temperature as 1000.degree. C. or so for the provision of diodes for high speed applications.
We object to the gold diffusion at such high temperatures because of the resulting production of a multiplicity of minute zones of gold-silicon (Au--Si) alloy at the interface between the semiconductor substrate and the gold layer. The Au--Si zones have persisted after the gold layer has been etched away, with the consequent creation of irregularities on the exposed surface of the substrate. It has therefore been difficult to form an electrode of desired physical properties on this surface of the substrate. An increase in forward voltage has been unavoidable. The surface irregularities have also caused uneven current distribution, so that the diodes fabricated by this known method have been susceptible to breakdown by current surges. Additionally, considerable leakage current has been present when the diode is reverse biased.
We have also ascertained that the Au--Si zones and the resulting surface roughness of the semiconductor substrate, which are both causes of the various inconveniences listed above, become more and more pronounced with an increase in the temperature at which gold is diffused. Such inconveniences would, of course, be mitigated by lowering the temperature of gold diffusion. This represents no acceptable solution at all because then the diffused gold would less serve the purpose of reducing the minority carrier lifetime.
Another known solution is found in Japanese Unexamined Patent Publication No. 54-55372 laid open to public inspection on May 2, 1979. It teaches to diffuse gold into semiconductor wafers at temperatures above 1000.degree. and subsequently to heat the wafers to 600.degree. to 700.degree. C. in order to provide desired reverse current and switching speed characteristics. It is, however, silent on the subject of how to reduce the Au--Si zones and resulting irregularities on the surface of the wafers.